1. Field of the Invention
The present invention relates to a method for heating a patterned substrate, and particularly to a method for heating the patterned substrate in reflow soldering process.
2. Description of the Related Art
In high integration mounting of electronic components on a printed circuit board substrate (hereinafter substrate), a reflow soldering process is becoming widely used in recent years. The reflow soldering process, apart from the conventional flow-process to dip a substrate with the electronic components thereon into a solder bath, is carried out by: mounting electronic component on a patterned substrate, which has been previously coated with a creamy solder paste containing fine solder powder and paste, at predetermined locations on the substrate; and subsequently irradiating the substrate by infra-red rays to heat the fine powder of solder and fuse the mounted components onto the conductive pattern on the substrate.
A conventional general reflow furnace is shown in FIG. 10, wherein on a belt conveyor moving at a constant speed v carries a sequence of substrates 2 having electronic components (not shown) thereon being heated by infra-red heater 3. A first part A of the furnace is a pre-heating part for heating the substrates 2 to about 160.degree. C. A second part B of the furnace is a soldering part for heating the substrates 2 to about 250.degree. C. to fuse the powder solder and thereby effect the soldering. The substrates 2 are thereafter cooled by a blower (not shown) and taken from the conveyor.
The above-mentioned conventional reflow substrate heating process does not necessarily provide satisfactory heating efficiency of the substrate. That is, because of differences in thermal capacity and because of nonuniform arrangements of the electronic components on the substrate, there arises temperature nonuniformity or undesirable temperature distribution. Accordingly, if the heating condition of the infra-red furnace is designed to meet the case of the electronic components of smaller heat capacities, the electronic components of larger heat capacities are heated insufficiently, thereby resulting in undesirably low temperature, resulting in insufficient soldering of the latter components. On the other hand, if the heating of the furnace is designed to meet the heat capacity of the electronic components of the larger heat capacities the components of the smaller heat capacities are excessively heated thereby resulting in thermal destruction, melting or deformation. As a conclusion, when electronic components having different heat capacities are intermixed on the substrate, the conventional reflow process may not appropriately heat the respective components to their best heating conditions at the same time.
In order to solve the above-mentioned problem, a proposal to heat the substrate with electronic components thereon, under a masking, has been disclosed in the Japanese Patent Kokai (Unexamined published patent application) Sho 60-208893. However, the method disclosed in the Japanese Patent Kokai Sho 60-208893 fails to sufficiently control the irradiation power to meet the irregularity of actual heat capacities of the components mounted on the substrate, but only applies larger slits for components of larger heat capacity and smaller slits for components of smaller heat capacity. This Kokai does not disclose concrete means to select nor define the slit size of the mask, nor does Kokai disclose a method of assessing heat capacities of the respective components for optimum selection of the irradiation control. Kokai is thus not able to predict a secondary change of temperature distribution due to secondary reaction of heat conduction on or in the substrate, and hence the design of the heat control mask has to be made in an imprecise trial and error manner.